1. Technical Field of the Invention
The present invention generally relates to methods and apparatus for recovering elemental sulfur from hydrogen sulfide-containing gas streams, and more particularly to such methods and apparatus that increase the sulfur recovery capacity of new or existing Claus plants and reduce the size and complexity of such plants.
2. Description of Related Art
Large quantities of H2S-containing gases are commonly produced in the petrochemical industry by amine treating units and sour water stripping units. Claus sulfur recovery plants (“Claus plants”) are in widespread use to convert this environmentally hazardous H2S to useful elemental sulfur by oxidation according to the overall or net equationH2S+½O2→1/xSx+H2O  (1)wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure. The net production of elemental sulfur is usually accomplished as a series of process steps carried out according to a conventional plant flow scheme. First, a free flame combustion step takes place by burning ⅓ of the H2S in a free flame combustion chamber or burner according to the equationH2S+ 3/2O2→SO2+H2O  (2).Oxygen for the combustion stage is usually supplied by air from an air compressor or blower. The combustion stage is followed by the “Claus” stage, in which the “Claus reaction” takes place according to the equation2H2S+SO23/xSx+2H2O  (3)wherein x=2, 6 or 8, depending on the particular conditions of temperature and pressure. The Claus reaction initially takes place in the reaction furnace immediately following the burner, and while the gases are at near-flame temperatures. After the gases exit the reaction furnace they are cooled in a heat exchanger, usually with boiling water circulating in the heat exchanger and being converted to medium to high-pressure steam. After cooling, the gases are cooled further in another heat exchanger (sulfur condenser), in which boiling water is circulated to make low pressure steam. At this stage in the process about 50-70% of the incoming H2S will typically have been converted to elemental sulfur. The actual amount depends on such factors as inlet H2S concentration, flame temperature, residence time in the reaction furnace following the burner, and the presence and amount of other chemicals such as other combustibles or carbon dioxide. Condensed liquid sulfur product is usually recovered at this point in the process.
A 70% level of conversion is insufficient by today's standards to allow the effluent from the Claus furnace to be emitted to the atmosphere or to make tail gas treatment economical at this point. An increase in the overall level of conversion is usually achieved by removing one of the reaction products from the mixture (e.g., by condensing and removing liquid elemental sulfur), and then allowing the remaining gases to continue reacting until equilibrium is reached (Equation 3). After the reaction furnace, the reacted gases are cooled in a heat exchanger against boiling water. The gases can be cooled to allow condensation of sulfur in this heat exchanger, or, more typically, the cooled gases from the heat exchanger are further cooled in a separate heat exchanger/sulfur condenser to facilitate condensation of the sulfur formed in the first reaction stage.
In modified Claus plants, further recovery of sulfur is accomplished by taking the gases from the first condenser, reheating, and then passing the gases over a high surface area Claus catalyst in a packed bed reactor. The Claus reaction (Equation 3) takes place on the catalyst up to the equilibrium limit of the reaction. Some well-known Claus catalysts are bauxite, alumina and titania The Claus catalytic reactors are normally operated in the gas phase to prevent condensed sulfur from plugging the pores of the catalyst. To enhance recovery of sulfur via the Claus reaction, the elemental sulfur is conventionally removed by condensation in a sulfur condenser which follows the catalytic reactor. Similar reheat, reaction and condensation steps are commonly repeated two to three times in order to maximize sulfur yield of the plant. Because of the equilibrium restraints inherent in the Claus reaction (Equation 3), adding more catalytic Claus reactors becomes ineffective beyond a total of three or four units, so other measures must be taken in order to further increase sulfur recovery beyond about 98 vol. % of the initial H2S and to complete the recovery of the remaining sulfur before the effluent is released to the atmosphere.
The addition of equipment needed to improve recovery almost invariably decreases the capacity of the plant by adding resistance to flow from additional friction. Thus the addition of each catalytic Claus reactor, heater, condenser and tail gas treatment unit is accompanied by a reduction in operating pressure. Moreover, as demand for sulfur recovery capacity grows in an existing facility, the flows of O2-containing gas and H2S-containing gas into the Claus plant will increase. This increase in flow causes an increase in pressure drop through the system approximated by the relationshipDP2/DP1=(Q2/Q1)2  (4)
where DP is pressure drop, Q is volumetric flow rate, 1 is the initial flow condition, and 2 is the new flow condition. In any given system, at a certain flow rate of H2S-containing gas the pressure drop due to friction from flow will exceed the available pressure drop through the unit. At that point, the unit is capacity constrained. Conventional Claus plants operate at low pressure, usually 20-30 psia at the front of the plant. In almost every case, a conventional sulfur recovery plant with a burner, reaction furnace, single catalytic Claus reactor, and single tail gas treatment unit is limited to 5 to 15 psi of available pressure drop. Many existing Claus plants suffer from a severe constraint in capacity.
Cost efficient ways to meet the ever increasing need for greater sulfur recovery capacity are sought. U.S. Pat. No. 6,776,974 (Monsanto Enviro-Chem Systems, Inc.) describes some proposed solutions that are intended to decrease the size and increase the capacity of Claus installations. Some approaches involve combusting the tail gas to oxidize the residual sulfur species to sulfur dioxide, and then recovering and recycling the resulting concentrated stream of sulfur dioxide to a point upstream of the Claus unit. Another approach includes contacting an acid gas feed stream and sulfur dioxide with a Claus conversion catalyst in a single Claus catalytic reaction zone to form elemental sulfur and water. A drawback of the latter approach is that the SO2 feed must provided in addition to the H2S feed. This is usually done by burning sulfur or H2S, which necessitates a reaction furnace and possibly a waste heat boiler. Another downside of directly contacting a Claus conversion catalyst with the acid gas feed stream is that serious negative consequences can result when the H2S stream contains certain additional components. In the case of hydrocarbon and ammonia contaminants, for example, incomplete destruction of ammonia and incomplete combustion of hydrocarbon typically occur, leading to ammonia salt plugging, and sooting (coke deposition).
Another avenue for expanding sulfur recovery plant capacity is to increase the available pressure drop for the above-described increased friction losses that can occur due to greater flow. This is accomplished by raising the air blower discharge pressure, increasing the operating pressure of the source that provides the H2S-containing gas (e.g., amine regenerator or sour water stripper), or increasing the pressure holding capacity of the sulfur liquid seal devices. The difficulty of replacing underground sulfur sealing devices and the poor operation of feed-producing units at higher pressure usually limits the gains that can be realized from this latter approach.
Still another way to increase plant capacity is to reduce the overall flow of fluid by using molecular oxygen instead of air in the combustion stage. Elimination of the nitrogen component of air reduces the total flow per unit volume of H2S-containing gas and allows more H2S to be processed for the same pressure drop limitation. The economic appeal of oxygen enrichment is limited by the cost of the oxygen, the temperatures attained in an O2-enriched flame, and by the cost of the special equipment needed to handle high concentrations of oxygen.
Efforts to redesign the system for low pressure drop have generally met with little success, since each piece of equipment must be designed for low pressure drop and the total number of pieces of equipment in a Claus Plant, there is only a small amount of pressure drop available for the entire plant. The design of the heat exchanger that follows the reaction furnace is a major factor in the plant's overall performance. Key parameters in the design of waste heat exchangers have been previously discussed by W. P. Knight (“Evaluate waste heat steam generators,” Hydrocarbon Processing, July 1978, Gulf Publishing Co., Houston, Tex., pp. 126-130). Because the gases in the process are corrosive to carbon steel under typical conditions, the tube walls must be kept close to the temperature of the boiling water on the shell side of the exchanger to prevent rapid corrosion and loss of containment on the tube side. Thus, a favorable heat transfer coefficient is necessary. A low heat transfer coefficient prompts an increase in the heat exchange areas in the design, usually by lengthening the tubes of the heat exchanger, with an associated increase in pressure drop due to the increased friction from the longer tubes. One concern when using longer tubes is that elemental sulfur can form tenacious deposits that can plug equipment, if allowed to solidify within the system. While greater shell diameters and an increased number of tubes could be used to overcome the pressure drop restriction, those remedies tend to greatly increase the cost of heat exchanger construction compared to merely lengthening the existing number of tubes. The larger diameters and limits on tube mass velocities generally lead to undesirably low heat transfer coefficients. These and other considerations have tended to lead heat exchanger designers toward larger diameter tubes than is customary in most petrochemical plant heat exchanger services, for instance.
There are also design concerns with respect to the sulfur condensers, including such factors as velocity of the sulfur bearing gas stream, fogging, heat transfer, tube selection, mechanical design, and arrangement (W. P. Knight, “Improve sulfur condensers,” Hydrocarbon Processing, May 1978, Gulf Publishing Co., Houston, Tex., vol. 57 (No. 5) pp. 239-241; Laurance Reid, Gas Conditioning Conference Fundamentals—Sulfur Recovery, 2003, Norman, O K, p. 80-81.) The paradigm of present day sulfur condenser design generally includes: (1) controlling the overall heat transfer coefficient between 8 and 18 BTU/hr/ft2/° F.; (2) controlling the mass velocity in the tube below 6.1 lbm/ft2/sec and greater than about 2 lbm/ft2/sec; (3) sloping the tubes toward the outlet at approximately ⅛ inch drop per foot of length; and (4) avoiding vertical or absolutely horizontal tubes. Following those guidelines will deter loss of elemental sulfur from the condenser, by mechanisms of entrainment and fogging, to subsequent reaction stages, and thereby improves the overall efficiency of a Claus unit. The efficiency of any sulfur condenser is inherently limited by the properties of elemental sulfur. In order for sulfur to condense from the vapor phase to the liquid phase, the sulfur in the vapor phase must be in equilibrium with sulfur in the liquid phase.
Designing a higher-capacity Claus plant is also complicated by the size and number of Claus catalytic reaction units that are customarily used today, each including a sulfur condenser with coolant system, reheater, and Claus conversion catalyst. Typical Claus catalyst beds are designed and industry data shows they are typically operated in a range of gas hourly space velocity (GHSV) of about 1000 h−1 to about 1500 h−1. The GHSV is usually represented at standard cubic feet/hour of incoming gases to the reactor divided by the cubic feet of catalyst in the reactor. The nominal design GHSV of a Claus catalytic reactor can make the catalyst volume requirements quite large. To reduce the frictional pressure loss through such a large bed, the catalyst bed is usually designed with large cross-sectional area and short catalyst depth, compared to other fixed bed catalytic reaction systems.
It is also important in the design of a Claus sulfur recovery plant to remove heat from the process gases after each reaction step. The design of the waste heat exchanger or boiler following the burner/furnace (thermal) zone and the subsequent sulfur condensers of a conventional sulfur recovery plant is a major factor in the overall performance of a sulfur plant. A generally accepted design principle for waste heat boilers is to control the heat flux through the tube wall below about 30,000 BTU/hr/ft2. This recommended level prevents overheating the tube by keeping the outside of the tube wall wetted with water. Combining the design elements of multiple reheat, reaction and condensation stages with the GHSV requirements for a typical Claus reactor leads to very large, expensive units.
There remains a pressing need for improvement of existing Claus sulfur recovery plants, and for new, higher-capacity, compact installations, in order to meet the increasing burden of handling greater volumes of more concentrated acid gas streams. At the same time, there is also a necessity to limit construction costs and operating expenses of new and existing Claus sulfur recovery plants. There is also a need for more productive sulfur recovery processes that can meet the ever more stringent standards for release of residual sulfur compounds into the environment.